Partition by is used while writing the analytical functions in Oracle.
Referring to this blog - http://www.orafaq.com/node/55
It is true that whatever an analytic function does can be done by native SQL, with join and sub-queries. But the same routine done by analytic function is always faster, or at least as fast, when compared to native SQL.
The general syntax of analytic function is:
Function(arg1,..., argn) OVER ( [PARTITION BY <...>] [ORDER BY <....>] [] )
is like "ROW " or "RANK "
This brings out the main difference between aggregate and analytic functions. Though analytic functions give aggregate result they do not group the result set. They return the group value multiple times with each record. As such any other non-"group by" column or expression can be present in the select clause, for example, the column EMPNO in Query-2.
Analytic functions are computed after all joins, WHERE clause, GROUP BY and HAVING are computed on the query. The main ORDER BY clause of the query operates after the analytic functions. So analytic functions can only appear in the select list and in the main ORDER BY clause of the query.
In absence of any PARTITION or inside the OVER(
) portion, the function acts on entire record set returned by the where
clause. Note the results of Query-3 and compare it with the result of aggregate function query Query-4.
Some functions support the inside the partition
to further limit the records they act on. In the absence of any
analytic functions are computed on all the records
of the partition clause.
The functions SUM, COUNT, AVG, MIN, MAX are the common analytic functions the result of which does not depend on the order of the records.
Functions like LEAD, LAG, RANK, DENSE_RANK, ROW_NUMBER, FIRST, FIRST VALUE, LAST, LAST VALUE depends on order of records. In the next example we will see how to specify that.
The general syntax of specifying the ORDER BY clause in analytic function is:
ORDER BY [ASC or DESC] NULLS [FIRST or LAST]
The syntax is self-explanatory.
ROW_NUMBER( ) gives a running serial number to a partition of records. It is very useful in reporting, especially in places where different partitions have their own serial numbers. In Query-5, the function ROW_NUMBER( ) is used to give separate sets of running serial to employees of departments 10 and 20 based on their HIREDATE.
Query-6 shows the usage of both RANK and DENSE_RANK. For DEPTNO 20 there are two contenders for the first position (EMPNO 7788 and 7902). Both RANK and DENSE_RANK declares them as joint toppers. RANK skips the next value that is 2 and next employee EMPNO 7566 is given the position 3. For DENSE_RANK there are no such gaps.
LEAD (, , ) OVER ()
is the expression to compute from the leading row.
is the index of the leading row relative to the current row.
is a positive integer with default 1.
is the value to return if the points to a row outside the partition range.
The syntax of LAG is similar except that the offset for LAG goes into the previous rows.
Query-7 and its result show simple usage of LAG and LEAD function.
FIRST_VALUE() OVER ()
The FIRST_VALUE analytic function picks the first record from the partition after doing the ORDER BY. The is computed on
the columns of this first record and results are returned. The
LAST_VALUE function is used in similar context except that it acts on
the last record of the partition.
The general syntax is:
Function( ) KEEP (DENSE_RANK FIRST ORDER BY) OVER ()
Please note that FIRST and LAST are the only functions that deviate from the general syntax of analytic functions. They do not have the ORDER BY inside the OVER clause. Neither do they support any clause. The ranking done in FIRST and LAST is always
DENSE_RANK. The query below shows the usage of FIRST function. The LAST
function is used in similar context to perform computations on last
ranked records.
The general syntax of the is
[ROW or RANGE] BETWEEN AND
can be any one of the following
For RANGE type windows the definition is in terms of values before or after the current ORDER. We will take this up in details latter.
The ROW or RANGE window cannot appear together in one OVER clause. The window clause is defined in terms of the current row. But may or may not include the current row. The start point of the window and the end point of the window can finish before the current row or after the current row. Only start point cannot come after the end point of the window. In case any point of the window is undefined the default is UNBOUNDED PRECEDING for and UNBOUNDED FOLLOWING for
.
If the end point is the current row, syntax only in terms of the start point can be can be
[ROW or RANGE] [ PRECEDING or UNBOUNDED PRECEDING ]
[ROW or RANGE] CURRENT ROW is also allowed but this is redundant. In this case the function behaves as a single-row function and acts only on the current row.
Referring to this blog - http://www.orafaq.com/node/55
It is true that whatever an analytic function does can be done by native SQL, with join and sub-queries. But the same routine done by analytic function is always faster, or at least as fast, when compared to native SQL.
The general syntax of analytic function is:
Function(arg1,..., argn) OVER ( [PARTITION BY <...>] [ORDER BY <....>] [
How are analytic functions different from group or aggregate functions?
SELECT deptno, COUNT(*) DEPT_COUNT FROM emp WHERE deptno IN (20, 30) GROUP BY deptno; DEPTNO DEPT_COUNT ---------------------- ---------------------- 20 5 30 6 2 rows selected
Query-1
Consider the Query-1 and its result. Query-1
returns departments and their employee count. Most importantly it
groups the records into departments in accordance with the GROUP BY
clause. As such any non-"group by" column is not allowed in the select
clause.SELECT empno, deptno,
COUNT(*) OVER (PARTITION BY
deptno) DEPT_COUNT
FROM emp
WHERE deptno IN (20, 30);
EMPNO DEPTNO DEPT_COUNT
---------- ---------- ----------
7369 20 5
7566 20 5
7788 20 5
7902 20 5
7876 20 5
7499 30 6
7900 30 6
7844 30 6
7698 30 6
7654 30 6
7521 30 6
11 rows selected.
Query-2
Now consider the analytic function query (Query-2) and its result. Note the repeating values of DEPT_COUNT column.This brings out the main difference between aggregate and analytic functions. Though analytic functions give aggregate result they do not group the result set. They return the group value multiple times with each record. As such any other non-"group by" column or expression can be present in the select clause, for example, the column EMPNO in Query-2.
Analytic functions are computed after all joins, WHERE clause, GROUP BY and HAVING are computed on the query. The main ORDER BY clause of the query operates after the analytic functions. So analytic functions can only appear in the select list and in the main ORDER BY clause of the query.
In absence of any PARTITION or
SELECT empno, deptno,
COUNT(*) OVER ( ) CNT
FROM emp
WHERE deptno IN (10, 20)
ORDER BY 2, 1;
EMPNO DEPTNO CNT
---------- ---------- ----------
7782 10 8
7839 10 8
7934 10 8
7369 20 8
7566 20 8
7788 20 8
7876 20 8
7902 20 8
Query-3
SELECT COUNT(*) FROM emp
WHERE deptno IN (10, 20);
COUNT(*)
----------
8
Query-4
How to break the result set in groups or partitions?
It might be obvious from the previous example that the clause PARTITION BY is used to break the result set into groups. PARTITION BY can take any non-analytic SQL expression.Some functions support the
The functions SUM, COUNT, AVG, MIN, MAX are the common analytic functions the result of which does not depend on the order of the records.
Functions like LEAD, LAG, RANK, DENSE_RANK, ROW_NUMBER, FIRST, FIRST VALUE, LAST, LAST VALUE depends on order of records. In the next example we will see how to specify that.
How to specify the order of the records in the partition?
The answer is simple, by the "ORDER BY" clause inside the OVER( ) clause. This is different from the ORDER BY clause of the main query which comes after WHERE. In this section we go ahead and introduce each of the very useful functions LEAD, LAG, RANK, DENSE_RANK, ROW_NUMBER, FIRST, FIRST VALUE, LAST, LAST VALUE and show how each depend on the order of the record.The general syntax of specifying the ORDER BY clause in analytic function is:
ORDER BY
The syntax is self-explanatory.
ROW_NUMBER, RANK and DENSE_RANK
All the above three functions assign integer values to the rows depending on their order. That is the reason of clubbing them together.ROW_NUMBER( ) gives a running serial number to a partition of records. It is very useful in reporting, especially in places where different partitions have their own serial numbers. In Query-5, the function ROW_NUMBER( ) is used to give separate sets of running serial to employees of departments 10 and 20 based on their HIREDATE.
SELECT empno, deptno, hiredate, ROW_NUMBER( ) OVER (PARTITION BY deptno ORDER BY hiredate NULLS LAST) SRLNO FROM emp WHERE deptno IN (10, 20) ORDER BY deptno, SRLNO; EMPNO DEPTNO HIREDATE SRLNO ------ ------- --------- ---------- 7782 10 09-JUN-81 1 7839 10 17-NOV-81 2 7934 10 23-JAN-82 3 7369 20 17-DEC-80 1 7566 20 02-APR-81 2 7902 20 03-DEC-81 3 7788 20 09-DEC-82 4 7876 20 12-JAN-83 5 8 rows selected.
Query-5 (ROW_NUMBER example)
RANK and DENSE_RANK both provide rank to the records based on some
column value or expression. In case of a tie of 2 records at position N,
RANK declares 2 positions N and skips position N+1 and gives position
N+2 to the next record. While DENSE_RANK declares 2 positions N but does
not skip position N+1.Query-6 shows the usage of both RANK and DENSE_RANK. For DEPTNO 20 there are two contenders for the first position (EMPNO 7788 and 7902). Both RANK and DENSE_RANK declares them as joint toppers. RANK skips the next value that is 2 and next employee EMPNO 7566 is given the position 3. For DENSE_RANK there are no such gaps.
SELECT empno, deptno, sal, RANK() OVER (PARTITION BY deptno ORDER BY sal DESC NULLS LAST) RANK, DENSE_RANK() OVER (PARTITION BY deptno ORDER BY sal DESC NULLS LAST) DENSE_RANK FROM emp WHERE deptno IN (10, 20) ORDER BY 2, RANK; EMPNO DEPTNO SAL RANK DENSE_RANK ------ ------- ----- ----- ---------- 7839 10 5000 1 1 7782 10 2450 2 2 7934 10 1300 3 3 7788 20 3000 1 1 7902 20 3000 1 1 7566 20 2975 3 2 7876 20 1100 4 3 7369 20 800 5 4 8 rows selected.
Query-6 (RANK and DENSE_RANK example)
LEAD and LAG
LEAD has the ability to compute an expression on the next rows (rows which are going to come after the current row) and return the value to the current row. The general syntax of LEAD is shown below:LEAD (
The syntax of LAG is similar except that the offset for LAG goes into the previous rows.
Query-7 and its result show simple usage of LAG and LEAD function.
SELECT deptno, empno, sal,
LEAD(sal, 1, 0) OVER (PARTITION BY dept ORDER BY sal DESC NULLS LAST) NEXT_LOWER_SAL,
LAG(sal, 1, 0) OVER (PARTITION BY dept ORDER BY sal DESC NULLS LAST) PREV_HIGHER_SAL
FROM emp
WHERE deptno IN (10, 20)
ORDER BY deptno, sal DESC;
DEPTNO EMPNO SAL NEXT_LOWER_SAL PREV_HIGHER_SAL
------- ------ ----- -------------- ---------------
10 7839 5000 2450 0
10 7782 2450 1300 5000
10 7934 1300 0 2450
20 7788 3000 3000 0
20 7902 3000 2975 3000
20 7566 2975 1100 3000
20 7876 1100 800 2975
20 7369 800 0 1100
8 rows selected.
Query-7 (LEAD and LAG)
FIRST VALUE and LAST VALUE function
The general syntax is:FIRST_VALUE(
The FIRST_VALUE analytic function picks the first record from the partition after doing the ORDER BY. The
-- How many days after the first hire of each department were the next
-- employees hired?
SELECT empno, deptno, hiredate ? FIRST_VALUE(hiredate)
OVER (PARTITION BY deptno ORDER BY hiredate) DAY_GAP
FROM emp
WHERE deptno IN (20, 30)
ORDER BY deptno, DAY_GAP;
EMPNO DEPTNO DAY_GAP
---------- ---------- ----------
7369 20 0
7566 20 106
7902 20 351
7788 20 722
7876 20 756
7499 30 0
7521 30 2
7698 30 70
7844 30 200
7654 30 220
7900 30 286
11 rows selected.
Query-8 (FIRST_VALUE)
FIRST and LAST function
The FIRST function (or more properly KEEP FIRST function) is used in a very special situation. Suppose we rank a group of record and found several records in the first rank. Now we want to apply an aggregate function on the records of the first rank. KEEP FIRST enables that.The general syntax is:
Function( ) KEEP (DENSE_RANK FIRST ORDER BY
Please note that FIRST and LAST are the only functions that deviate from the general syntax of analytic functions. They do not have the ORDER BY inside the OVER clause. Neither do they support any
-- How each employee's salary compare with the average salary of the first
-- year hires of their department?
SELECT empno, deptno, TO_CHAR(hiredate,'YYYY') HIRE_YR, sal,
TRUNC(
AVG(sal) KEEP (DENSE_RANK FIRST
ORDER BY TO_CHAR(hiredate,'YYYY') )
OVER (PARTITION BY deptno)
) AVG_SAL_YR1_HIRE
FROM emp
WHERE deptno IN (20, 10)
ORDER BY deptno, empno, HIRE_YR;
EMPNO DEPTNO HIRE SAL AVG_SAL_YR1_HIRE
---------- ---------- ---- ---------- ----------------
7782 10 1981 2450 3725
7839 10 1981 5000 3725
7934 10 1982 1300 3725
7369 20 1980 800 800
7566 20 1981 2975 800
7788 20 1982 3000 800
7876 20 1983 1100 800
7902 20 1981 3000 800
8 rows selected.
Query-9 (KEEP FIRST)
How to specify the Window clause (ROW type or RANGE type windows)?
Some analytic functions (AVG, COUNT, FIRST_VALUE, LAST_VALUE, MAX, MIN and SUM among the ones we discussed) can take a window clause to further sub-partition the result and apply the analytic function. An important feature of the windowing clause is that it is dynamic in nature.The general syntax of the
[ROW or RANGE] BETWEEN
- UNBOUNDED PECEDING
- CURRENT ROW
- <sql_expr> PRECEDING or FOLLOWING.
- UNBOUNDED FOLLOWING or
- CURRENT ROW or
- <sql_expr> PRECEDING or FOLLOWING.
For RANGE type windows the definition is in terms of values before or after the current ORDER. We will take this up in details latter.
The ROW or RANGE window cannot appear together in one OVER clause. The window clause is defined in terms of the current row. But may or may not include the current row. The start point of the window and the end point of the window can finish before the current row or after the current row. Only start point cannot come after the end point of the window. In case any point of the window is undefined the default is UNBOUNDED PRECEDING for
If the end point is the current row, syntax only in terms of the start point can be can be
[ROW or RANGE] [
[ROW or RANGE] CURRENT ROW is also allowed but this is redundant. In this case the function behaves as a single-row function and acts only on the current row.
Tags
SQL Functions
Thanks! Hardcore SQL.
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